A turbomachine is understood to mean, for example, a steam turbine. A steam turbine usually has a rotatably mounted rotor and a casing which is arranged around the rotor. A flow duct is formed between the rotor and the inner casing. The casing in a steam turbine has to be able to fulfill a plurality of functions. Firstly, the guide blades in the flow duct are arranged on the casing and, secondly, the inner casing must withstand the pressure and temperatures of the flow medium for all load situations and special operating situations. In the case of a steam turbine, the flow medium is steam. Furthermore, the casing must be designed in such a way that supplies and discharges, which are also designated as bleeds, are possible. A further function which a casing must fulfill is the possibility that a shaft end can be led through the casing.
Under the high stresses, pressures and temperatures occurring during operation, it is necessary that the materials are suitably selected and the design is selected in such a way that mechanical integrity and functionality become possible. For this purpose, it is necessary to use high-grade materials, particularly in the region of the inflow and of the first guide blade grooves.
For applications at fresh steam temperatures of above 650° C., such as, for example, 700° C., nickel-based alloys are suitable, since they withstand the loads occurring at high temperatures. However, the use of such a nickel-based alloy entails new challenges. Thus, the costs of nickel-based alloys are comparatively high, and moreover the producibility of nickel-based alloys is limited, for example, because of the restricted possibility for casting. The result of this is that the use of nickel-based materials must be minimized. Furthermore, nickel-based materials are poor heat conductors. The temperature gradients across the wall thickness are therefore so rigid that thermal stresses are comparatively high. Further, account must be taken of the fact that, when nickel-based materials are used, the temperature difference between the inlet and the outlet of the steam turbine rises.
Various concepts are adapted at the present time for providing a steam turbine which is suitable for high temperatures and high pressures. Thus, it is known to incorporate an inner casing structure comprising a plurality of parts into an outer casing structure, according to the Article Y. Tanaka et al. “Advanced Design of Mitsubishi Large Steam Turbines”, Mitsubishi Heavy Industries, Power Gen Europe, 2003, Dusseldorf, May 6-8, 2003.
It is also known to produce an inner casing from two parts according to DE 10 2006 027 237 A1.
A multi-component inner casing structure is likewise disclosed in DE 342 1067 and in DE 103 53 451 A1.